Vehicle emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations in a fuel vapor canister, and then purge the stored vapors during a subsequent engine operation. The stored vapors may be routed to engine intake for combustion, further improving fuel economy.
In a typical canister purge operation, a canister purge valve coupled between the engine intake and the fuel canister is opened, allowing for intake manifold vacuum to be applied to the fuel canister. Simultaneously, a canister vent valve coupled between the fuel canister and atmosphere is opened, allowing for fresh air to enter the canister. This configuration facilitates desorption of stored fuel vapors from the adsorbent material in the canister, regenerating the adsorbent material for further fuel vapor adsorption.
However, current and future engine systems may be configured to operate under relatively low manifold vacuum conditions. While this may increase engine efficiency, it also reduces the opportunities for fuel vapor canister purging. This may particularly apply to hybrid vehicles, which have a limited engine run time to begin with. As such, stored vapors may be prone to desorption during diurnal cycles, increasing vehicle emissions and failing to comply with government regulations.
The inventors herein have realized the above issues and have developed systems and methods to at least partially address these issues. In one example, a method for purging fuel vapors, comprising: purging fuel tank vapors directly from a fuel tank to an engine intake, bypassing a canister, via a venturi, while drawing canister vapors via the venturi into the purged fuel tank vapors en route to the engine intake. In this way, fuel tank vapors may be used to enable purging of a fuel vapor canister, even under conditions where there is insufficient manifold vacuum to enable a canister purge routine. By increasing the frequency of purge opportunities, bleed emissions from a saturated canister may be reduced.
In another example, a system for an evaporative emissions system, comprising: a fuel tank coupled to a fuel vapor canister via a first fuel tank isolation valve; an ejector coupled to the fuel vapor canister and a second fuel tank isolation valve, the second fuel tank isolation valve configured to: responsive to a fuel tank pressure being above a threshold, enable fuel vapor to flow from the fuel tank through the ejector to an engine intake; and draw a vacuum on the fuel vapor canister. In this way, the system leverages fuel tank vapor pressure, which currently has no benefits, into generating a vacuum applied to a fuel vapor canister. The vacuum generated by venting fuel tank vapor through the ejector does not add any additional load on the engine.
In yet another example, a method for purging a fuel vapor canister, comprising: during a first condition including a fuel tank pressure above a threshold, close a first fuel tank isolation valve, the first fuel tank isolation valve coupled between a fuel tank and a fuel vapor canister; open a second fuel tank isolation valve, the second fuel tank isolation valve coupled between the fuel tank, the fuel vapor canister, and an engine intake; and open a canister purge valve and canister vent valve. In this way, fuel tank vapors may be purged directly to intake under some conditions, while drawing manifold vacuum is not required to purge the fuel vapor canister. This may lead to an increase in engine efficiency, as a high intake vacuum is not required to purge the fuel vapor canister in order to comply with government regulations for evaporative emissions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.